1. Field of the Invention
The present invention relates to a camera system for transmitting and receiving lens data between a camera body and a photographing lens.
2. Description of Related Art
A photographing lens, also referred to as an (interchangeable lens) in a recent single lens reflex camera having an automatic focusing device is usually provided with memory means (e.g., a ROM), in which lens data peculiar to the photographing lens is stored.
The camera body is provided with a microcomputer which communicates with the lens to read the lens data stored in the ROM in order to calculate exposure factors (i.e.; diaphragm value and shutter speed, etc.), based on the lens data and luminance of the object to be taken in accordance with a predetermined algorithm, to thereby carry out the exposure control and the focus control.
The lens data includes, for example, an Apex value Avmin (open F-number) at an open aperture, an Apex value Avmax (maximum F-number) at a minimum aperture, a focal length f, or diaphragm data for the highest quality picture (best quality diaphragm data). The best quality diaphragm data includes, for example, the diaphragm value at which the highest resolution is given, the diaphragm value at which the highest modulation transfer function (MTF) is given, the diaphragm value at which there is no vignetting, or the diaphragm value at which the aperture defined by the diaphragm blades is substantially circular.
It is also known to provide a control means in the photographing lens to perform part of the control, as well as data calculation, which have typically been carried out by the camera body in conventional cameras. For instance, a microcomputer (e.g., CPU) is provided in the photographing lens to carry out the calculation of optimum diaphragm values or focal lengths, etc., the focus control, or the zooming, etc., in association with the microcomputer of the camera body. The calculated data and the control commands are transmitted to and received from both the microcomputer of the photographing lens and the microcomputer of the camera body through an intermittent communication between the microcomputers.
The microcomputer of the photographing lens is usually supplied with electrical power by a power source in the camera body. The microcomputer of the photographing lens automatically performs the operations mentioned above, such as calculation and communication whenever the power source of the camera body is turned ON, regardless of the photographing operation, resulting an increased power consumption. Furthermore, if the power to be supplied to the microcomputer of the photographing lens is removed, all the operations must again be carried out from the beginning, upon restarting. This results in an increase of communication time and a delay in operation.